I would not suggest slowly modifying the size data
buffers file over time, because then you will have
that set differently for different backups and when
you restore you will have to know what it was set to
at the time the backup ocurred. If you are going to
adjust the default size_data_buffers I would just take
it straight to the max recommended size for a Windows
server which is 65536. You can go as high as 128K on a
Windows 2000 server assuming your drive and HBA or
scsi card can handle it but if you are not sure I
would stick with the 65536. Here is a technote on
buffer tuning in Windows. 

http://seer.support.veritas.com/search_forms/SearchFrame.asp?SearchTerm=%22net%5fbuffer%5fsz%22&Path=seer%2esupport%2everitas%2ecom%2fdocs%2f262911%2ehtm

Thanks,

Jeff Redington

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"A wise man once said 'Wherever you go, there you are'"
      -Mike Brady